The present invention relates to a system for detecting slips of wheels of a four-wheel drive vehicle having a central differential, which is called a full time four-wheel drive vehicle.
In a four-wheel drive vehicle, slipping will occur either of front wheels or rear wheels, or all of four wheels. The slippings of front or rear wheels can be prevented by locking the central differential. However, since the differential operation is restrained, cornering ability is deteriorated. Further, when the central differential is locked, slipping of the wheels can not be detected, so that it is impossible to automatically control to release the lock of the differential. Accordingly, the differential is manually operated to release the lock at driving on a snowy road having a low friction coefficient .mu..
Recently, a torque split control system is provided for controlling the distribution ratio of torque of the engine to front and rear wheels, when front wheels or rear wheels slip. For example, if the front wheels slip, the torque split control system operates to increase the torque applied to the rear wheels, so that slipping of front wheels is prevented. Thus, the vehicle can be driven round a corner with the differential operation.
However, although two-wheel slip can be prevented by controlling the torque distribution ratio, slips of all of four wheels can not be prevented by the torque split control. In order to restrain the slipping of four wheels, for example, the power of the engine is reduced to decrease the traction.
Since a traction control system is operated in relation to slipping of the wheels, a slip detecting system is necessary. Since in the four-wheel drive vehicle, four wheels may slip at the same time, it is necessary to provide a four-wheel slip detector.
Japanese Utility Model Application Laid-Open No. 59-99827 discloses a slip detecting system in which a vehicle speed sensor and tire rotating speed sensors are provided for detecting slipping of a tire. However, the system can not detect slipping of four wheels.